Laser based printing systems are known in the art for producing hard copy reproductions of image data. In particular, U.S. Pat. No. 3,410,203, issued in the name of Fischbeck, discloses a non-impact printing apparatus which includes a hologram containing an image of the data to be printed on a printing surface sprinkled with toner. Light energy, such as that derived from a laser, is beamed onto the hologram to project a real image of the data to be printed onto the toner. The suggested laser source is said to comprise a gas laser such as a carbon dioxide (CO.sub.2) laser, which is described as particularly suitable because such a laser produces a light beam having a power level of up to one kilowatt. Such a high power level is selected so as to be capable of fusing a variety of toners onto the surface of the paper web surface in the shape of the printed data. Positioned above the paper web is a vacuum sweeper device which includes brushes that gather the excess unfused toner from the printing surface. The gathered toner is then removed by suction.
U.S. Pat. No. 3,780,214, issued in the name of Bestenreiner et al., discloses the production of color prints of multicolored originals on absorbent paper by laser exposure of recording layers, each of which consisting of a differently pigmented material, so that the resulting thermal images are representative of colored portions of the original. The thermal images are applied to a strip of paper so that the differently-colored images of the same original overlie each other. The recording layers can be provided on the paper strip or on discrete expendable or reusable flexible carriers which are transported across the paths of laser beams produced by a CO.sub.2 laser.
Similar laser-based printing or copying apparatus are disclosed in U.S. Pat. No. 3,601,484, issued in the name of Dybvig et al., and U.S. Pat. No. 4,148,057, issued in the name of Jesse.
These and other laser-based printing approaches suffer from several drawbacks. One is that the image recording material must be in the form of a unique and expendable carrier, such as the recording layers disclosed in Bestenreiner et al. Thus, the recording material is costly, difficult to use, and not commonly available.
Even if a simpler recording material is used, such as the toner disclosed in Fischbeck, the laser exposure must be quite long in duration, or intense, or both, to ensure adequate fusing of the material to the printing surface. This requirement exists in part due to the relatively high melting point of common toner materials, and in part due to the fact that the preferred toner removal systems, such as the vacuum sweeper device disclosed in Fischbeck, are quite aggressive in their action. Any partially-fused toner, even though it is a part of the image to be retained, will be removed by the preferred cleaning systems disclosed in prior art laser printing systems.
A gas laser is cited as the preferred exposure source because the above-described prior art printing approaches require an exposure source of considerable power output. However, gas lasers are more expensive and not nearly as compact as a semiconductor laser diode. Thus, the above-described prior art laser printing schemes cannot take advantage of the several benefits of semiconductor lasers (and other similarly compact and inexpensive laser sources).
Because prior art methods rely on a step of high power exposure of toner particles, followed by an aggressive cleaning step, the process is less responsive to images having high resolution or minute gradations of image contrast or density. Thus, the prior art laser printing methods described above utilize a recording process that is less sensitive than is desirable.
Further, prior art methods are not so amenable to cyclical operation at high processing speeds (high printing rates), wherein the time available for the exposure and cleaning steps must be reduced in order to make more prints in a given amount of time.